1. Field of the Invention
The invention relates generally to the field of floating petroleum drilling and production systems. More specifically, the invention relates to methods for retrieving part of a riser connecting a wellbore to a floating platform such that the platform can be moved in preparation for adverse weather conditions.
2. Background Art
Floating structures are known in the art for drilling wellbores in Earth formations located below the ocean floor, and for producing petroleum from such wellbores. The wellbores are typically drilled using fluid pressure control equipment, called a “blowout preventer” (BOP) affixed to the top of a casing cemented into a relatively shallow portion of the wellbore. A “riser”, which is a pipe formed from segments coupled end to end, is affixed to the top of the BOP and extends therefrom to the floating platform. The riser provides a conduit for fluids to move from the wellbore upwardly to the floating platform. Therefore a riser as used in wellbore drilling forms a conduit for drilling fluid and drill cuttings to be returned to the floating platform for processing and recirculation into the wellbore.
A riser is assembled to the wellbore from the floating platform by coupling together segments, called “joints” of riser, and moving the assembled “string” of joints of riser downward from the floating platform as successive riser joints are coupled to the string on the platform. The foregoing procedure continues until the riser is long enough to reach the wellbore from the floating platform, whereupon the lowermost end of the riser is coupled to the BOP.
During the approach of severely adverse weather conditions, such as a tropical cyclone (hurricane or typhoon), safety considerations require preparing the well for the possibility that the floating structure will be moved from its location, either by, or intentionally to avoid, the force of such weather conditions. In preparation for such weather conditions, it is necessary to retrieve the riser from the wellbore. Retrieving the riser includes lifting the riser and consecutively disassembling joints from the remaining riser string as it is suspended from the floating platform in the water. The disassembly continued until all the riser is disassembled and is retrieved from the water. Retrieving the riser is a time consuming and therefore costly procedure, the time and cost of which is related to the depth of water in which the well is being operated. Further, because of the amount of time needed to retrieve the riser, it is necessary to begin retrieval thereof at such a time prior to the expected arrival of such weather conditions as to make it more likely that the adverse weather conditions do not in fact approach the location of the floating platform. Thus, in a number of instances, retrieving the riser proves to be unnecessary.
A riser disconnect system and method disclosed in U S. Patent Application Publication No. 2004/0173356 A1 filed by Dore et al. provides a way to move a floating drilling structure from a well location by enabling disconnecting the riser from the well. Such system and method have particular application for wells drilled from floating drilling structures where the BOP is located at the top of the riser on the floating drilling structure. However, the method and system disclosed in the Dore et al. publication requires that a disconnect device for the riser be located proximate the water bottom in order to prevent the riser from collapsing under its own weight when tensile force from the floating drilling structure is thus removed. Therefore, the method and system disclosed in the Dore et al. publication still require retrieval of a substantial length of riser below the floating drilling structure, which is time consuming as explained above.
A riser system having a BOP near the water bottom and including a riser disconnect is shown in U.S. Pat. No. 5,657,823 issued to Kogure et al. The system shown in the '823 patent provides a way to disconnect the riser at a relatively shallow depth in the water, described as 50 to 500 feet, to enable relatively quickly moving the floating structure away from the well area if a storm approaches. The system shown in the '823 patent includes buoyancy devices such as canisters that are affixed to the lower portion of the riser in the event the upper portion riser is to be disconnected from the floating platform. When the upper portion of the riser is disconnected from the lower portion, the buoyancy devices support the lower portion of the riser in tension. It can be time consuming and difficult to affix buoyancy devices to a riser when it is deployed in the water.
What is needed is an improved method and system to reduce the amount of time needed to make adverse weather preparations for a floating drilling or production platform.